Satellite cell activity is differentially affected by contraction mode in human muscle following a work-matched bout of exercise

Abstract

Optimal repair and adaptation of skeletal muscle is facilitated by resident stem cells (satellite cells). To understand how different exercise modes influence satellite cell dynamics, we measured satellite cell activity in conjunction with markers of muscle damage and inflammation in human skeletal muscle following a single work- and intensity-matched bout of eccentric (ECC) or concentric contractions (CON). Participants completed a single bout of ECC (n = 7) or CON (n = 7) of the knee extensors. A muscle biopsy was obtained before and 24 h after exercise. Functional measures and immunohistochemical analyses were used to determine the extent of muscle damage and indices of satellite cell activity. Cytokine concentrations were measured using a multiplexed magnetic bead assay. Isokinetic peak torque decreased following ECC (p < 0.05) but not CON. Greater histological staining of the damage marker Xin was observed in muscle samples of ECC vs. CON. Tenasin C immunoreactivity increased 15 fold (p < 0.01) following ECC and was unchanged following CON. The inflammatory cytokines interferon gamma-induced protein 10 (IP-10) and monocyte chemotactic protein 1 (MCP-1) increased pre- to post-ECC (4.26 ± 1.4 vs. 10.49 ± 5.8 pg/ml, and 3.06 ± 0.7 vs. 6.25 ± 4.6 pg/ml, respectively; p < 0.05). There was no change in any cytokine post-CON. Satellite cell content increased 27% pre- to post-ECC (0.10 ± 0.031 vs. 0.127 ± 0.041, respectively; p < 0.05). There was no change in satellite cell number in CON (0.099 ± 0.027 vs. 0.102 ± 0.029, respectively). There was no fiber type-specific satellite cell response following either exercise mode. ECC but not CON resulted in an increase in MyoD positive nuclei per myofiber pre- to post-exercise (p < 0.05), but there was no change in MyoD DNA binding activity in either condition. In conclusion, ECC but not CON results in functional and histological evidence of muscle damage that is accompanied by increased satellite cell activity 24 h post-exercise.

Keywords: MyoD; Pax7; concentric; eccentric; resistance exercise; stem cell.

Figure 1

Prolonged isokinetic strength loss and muscle soreness indirectly indicates mild muscle damage following eccentric (ECC) but not concentric contractions (CON). (A) Mean isokinetic strength of the knee extensors in both ECC and CON groups pre-exercise, and each day for 4 days following exercise. Data are presented as a percentage of pre-exercise force production. (B) Mean muscle soreness of the knee extensor muscles in both ECC and CON groups pre-exercise, and each day for 4 days following exercise. Data are presented as mm on a 100 mm visual analog scale (VAS). Data are means ± SD. ^*Indicates significant difference (p < 0.05) from pre-exercise value. †Indicates significant difference between ECC and CON (p < 0.05).

Figure 2

Increased immunoreactivity and sarcolemmal localization of Xin suggests myofiber damage 24 h following eccentric (ECC) but not concentric contractions (CON). (A) Representative fluorescent images of a triple stained 8 μm section from pre- and 24 h post-exercise ECC and CON samples show increased immunoreactivity of Xin 24 h post-ECC. Scale bar = 20 μm. (B) Image of post-exercise ECC sample demonstrating the localization of Xin to the sarcolemma. White square in image denotes the boundaries of the enlarged image. Scale bar in merged image = 20 μm; scale bar in enlarged images = 10 μm.

Figure 3

Changes in pro-inflammatory cytokines following eccentric (ECC) but not concentric contractions (CON). Changes in MCP-1 (A) and IP-10 (B) concentrations pre- and 24 h post- exercise in muscle from subjects that performed a bout of ECC or CON. Data are means ± SD. ^*Indicates significant difference (p < 0.05) from all other conditions. †Indicates significant difference (p < 0.05) between pre- and post-exercise measures.

Figure 4

Increased immunoreactivity of tenascin C suggests extracellular matrix (ECM) de-adhesion 24 h following eccentric (ECC) but not concentric contractions (CON). (A) Representative fluorescent images of pre- and 24 h post-exercise ECC and CON samples. Scale bar = 100 μm. (B) Quantification of tenascin C immunoreactivity pre- and 24 h post-exercise for ECC and CON. Data are means ± SD. ^*Indicates significant difference (p < 0.01) from all other conditions.

Figure 5

Increased satellite cell content 24 h following eccentric (ECC) but not concentric contractions (CON). (A) Representative fluorescent image of a triple stained 8 μm section from an ECC sample for type I myosin heavy chain (red), Pax7 (green), and nuclei (DAPI; blue). White squares in merged images denote the boundaries of the enlarged images. Arrow points to type I associated satellite cell and arrowhead points to type II associated satellite cells. Scale bar in merged image = 50 μm; scale bar in enlarged images = 20 μm. (B) Quantification of Pax7 positive nuclei for individual subjects pre- and post-exercise for both ECC and CON. (C) Mean satellite cell data presented as the total number of Pax7⁺ cells per muscle fiber. (D) Number of Pax7⁺ nuclei associated with Type I fibers pre- and post-exercise for ECC and CON. (E) Number of Pax7⁺ nuclei associated with Type II fibers pre- and post-exercise for ECC and CON. Data are means ± SD. ^*Indicates significant difference (p < 0.05) between pre- and post-exercise measures.

Figure 6

DNA binding activity and nuclear accumulation of the myogenic regulatory factor MyoD 24 h following eccentric (ECC) or concentric contractions (CON). (A) DNA binding activity of MyoD from nuclear extract of muscle samples taken pre- and 24 h post-exercise for ECC and CON. (B) Image demonstrating a MyoD⁺ nucleus from a post-exercise ECC sample; and quantification of MyoD⁺ nuclei for subjects pre- and post-exercise for both ECC and CON. MyoD data are presented as the total number of MyoD⁺ cells per muscle fiber. Scale bar = 20 μm. Data are means ± SD. ^*Indicates significant difference (p < 0.05) from all conditions.